The present invention generally relates to focal plane arrays and more particularly to a focal plane array readout that employs a single capacitive feedback transimpedance amplifier for each column in the focal plane array.
Existing high performance direct readout devices for focal plane arrays generally require several transistors in each unit cell, a unit cell being the circuit that stores charge from the detector elements of the focal plane array. Typically the number of transistors employed in the unit cell is four. Consequently, it is generally not possible to build monolithic focal plane arrays with small unit cells and high fill factors, meaning the ratio of detector area to total unit cell area. For hybrid focal plane arrays, input circuit performance is generally compromised using such conventional circuits. Charge coupled device readouts also require large amounts of space in the unit cell, and therefore have similar drawbacks.
Readout devices have been developed which require only two transistors in the unit cell. Such devices are described in a publication entitled "A Solid State Color Video Camera with a Horizontal Readout MOS Imager," authored by Masaru Noda, et al, published in IEEE Transactions on consumer Electronics, Vol. CE-32, No. 3, August 1986. In addition to requiring more space in the unit cell this approach requires a very high speed amplifier with low noise, a combination that is very difficult to achieve in practice.
Single transistor unit cells have been used in the past for reading out photodiode arrays. However, the reported devices have employed signal extraction circuitry, such as feedback enhanced direct injection circuits, whose noise performance is somewhat less than desirable. Typical of such single transistor unit cells are those referenced in "Design Consideration and Performance of a New MOS Imaging Device," authored by Haruhisa Ando, et al, in IEEE Transactions on Electron Devices, Vol ED-32, No. 8, August 1985.